1. Field of the Invention:
The present invention relates to dye-donor elements for use according to thermal dye transfer methods, in particular to dye-donor elements comprising cyan dyes that have a good solubility in ecologically acceptable solvents and at the same time give high transfer densities and a good hue.
2. Description of the Prior art:
Thermal transfer methods have been developed to make prints from electronic pattern information signals e.g. from pictures that have been generated electronically by means of a colour video camera. To make such prints the electronic picture can be subjected to colour separation with the aid of colour filters. The different colour selections thus obtained can then be converted into electric signals, which can be processed to form cyan, magenta, and yellow electrical signals. The resulting electrical colour signals can then be transmitted to a thermal printer. To make the print a dye-donor element having repeated separate blocks of cyan, magenta, and yellow dye is placed in face-to-face contact with a receiving sheet and the resulting sandwich is inserted between a thermal printing head and a platen roller. The thermal printing head, which is provided with a plurality of juxtaposed heat-generating resistors, can selectively supply heat to the back of the dye-donor element. For that purpose it is heated up sequentially in correspondence with the cyan, magenta, and yellow electrical signals, so that dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which are in accordance with the pattern and intensity of the heat supplied to the dye-donor element.
The dye-donor element usually comprises a very thin support e.g. a polyester support, which is coated on both sides with an adhesive or subbing layer, one adhesive or subbing layer being covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion, the other adhesive layer at the opposite side of the support being covered with a dye/binder layer, which contains the printing dyes in a form that can be released in varying amounts depending on, as mentioned above, how much heat is applied to the dye-donor element.
The dye/binder layer comprising said repeated separate blocks of cyan, magenta, and yellow dye can be coated from a solution in appropriate solvents on the subbed support, but the known coating techniques are not quite adapted to the discontinuous repeated coating of three differently coloured dye/binder areas on said very thin support. It is therefore customary, especially in large-scale manufacturing conditions, to print said dye/binder layer on said support by printing techniques such as a gravure process.
In order to make possible an easy printing of the dye/binder composition on the support, this composition should comprise a good solvent to give it a printable ink-like nature.
The 2-carbamoyl-4-[N-(p-substituted aminoaryl)-imino]-1,4-naphthoquinone cyan dyes described in EP-A No. 227,096 have a favourable stability to light and a good hue, but they have the important drawback of having a solubility in common solvents like acetone, ethyl methyl ketone, and ethyl acetate that is too poor for being usable in the large-scale production of dye-donor elements according to the customary gravure printing techniques. The dyes should have a better solubility i.e. a solubility of at least 6% by weight, which means that one or more chlorinated hydrocarbon solvents such as methylene chloride, ethylene dichloride, and 1,1,2-trichloroethane have to be used as solvent or solvent mixture to render these cyan dyes printable.
However, from an ecological standpoint these chlorinated hydrocarbon solvents are being rejected nowadays and their recovery from a hot airstream used for drying the printed dye-donor element requires difficult and expensive techniques.
The similar 2-carbamoyl-4-[N-(p-substituted aminoaryl)-imino]-1,4-naphthoquinone dyes described in EP-A No. 270,677, EP-A No. 279,330, EP-A 285,665, JP-A 61-268,493, and JP-A 60-239,289 also have a poor solubility in common solvents and can thus hardly be printed according to customary gravure printing techniques.